This invention relates to methods and compositions for treating subterranean formations, and more specifically, to methods and compositions for breaking viscosified fluids utilized in the treatment of subterranean formations.
It is common practice to treat subterranean formations to increase the permeability or conductivity of such formations by procedures that are identified generally as fracturing processes. For example, it is a conventional practice to hydraulically fracture a well in order to create or enhance one or more cracks or “fractures” in the surrounding formation by mechanical breakdown of the formation. Fracturing may be carried out in wells that are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing, or other stimulation procedures, are production wells completed in oil and/or gas containing formations. However, disposal wells and injection wells used in secondary or tertiary recovery operations, for example, for the injection of water or gas, may also be fractured in order to facilitate the injection of fluids into such subterranean formations.
Hydraulic fracturing is accomplished by injecting a hydraulic fracturing fluid into the well and applying sufficient pressure on the fracturing fluid to cause the formation to break down with the attendant production of one or more fractures. Usually a gel, an emulsion or a foam having proppant particulates, such as sand or other particulate material, suspended therein is introduced into the fractures. The proppant particulates are deposited in the fractures and aid in maintaining the integrity of the fractures after the pressure is released and the fracturing fluid flows back into the well. The fracturing fluid usually has a sufficiently high viscosity to retain the proppant particulates in suspension or at least to reduce the tendency of the proppant particulates to settle out of the fracturing fluid as the fracturing fluid flows along the created fracture. Generally, a viscosifier such as a polysaccharide gelling agent is used to gel the fracturing fluid to provide the high viscosity needed to realize the maximum benefits from the fracturing process.
After the high viscosity fracturing fluid has been pumped into the formation and fracturing of the formation has occurred, it is desirable to remove the fluid from the formation to allow hydrocarbon production. Generally, the removal of the highly viscous fracturing fluid is realized by “breaking” the gel (i.e., reducing the viscosity of the fracturing fluid). Breaking the gelled fracturing fluid has commonly been accomplished by contacting the fracturing fluid with a suitable “breaker,” e.g., a viscosity-reducing agent.
Oxidizing breakers such as peroxides, persulfates, perborates, oxyacids of halogens and oxyanions of halogens, are used to break aqueous based fracturing or treating fluids at temperatures above 250° F. by oxidative depolymerization of the polymer backbone. However, in some temperature regimes these oxidizing agents can be generally ineffective for breaking the viscosity within a reasonable time period. For example, when using a chlorous acid oxidizing breaker below about 250° F., an activator is required to break the polymer in a timely fashion. Cupric ion chelated with ethylenediaminetetraacetic acid (EDTA) can provide the necessary activation; however, future use of transition metal ions, especially in oceanic drilling sites, may become less environmentally acceptable.